The Metamagic feature states in the Optional Features states the following:

Transmuted Spell
When you cast a spell that deals a type of damage from the following list, you can spend 1 sorcery point to change that damage type to one of the other listed types: acid, cold, fire, lightning, poison, thunder.

So will the damage persist for the duration of the spell or does the the statement "When you cast a spell" limit the effect's duration to the turn you cast the spell. My question for a ruling is related to spells that have a lingering effect.  For example, Wall of Fire; if a character uses the Transmuted Spell to change the  5d8 fire damage to thunder damage would the ruling be that the wall does 5d8 thunder damage for up to the minute duration? Or does the  5d8 thunder damage apply only to the creatures who must make a DEX save because they are in the area of the wall when the spell is cast, and then the following rounds any creature affected by the wall would take  5d8 fire damage.  

Appreciate any clarification and the reasoning used to establish a ruling on this.  

"When you cast a spell" is telling you when you make the decision to use the ability.

There's no duration or restriction on the damage type change, so it lasts for the entirety of the spell.